This invention relates to a retroreflective device which may be used wherever light reflection is desired. A principal application of the device is as a marker, and especially as a roadmarker, to provide directional guidance and therefore is described with respect to this use.
Roadmarkers are mounted on the surface of a roadway, such as along its center line or shoulders, to delineate paths or lanes for traffic, or at intersections to define stopping lines or cross-lanes for traffic, both vehicular and pedestrian. Markers of this type are mounted in spaced apart relation and serve to guide traffic in following or traversing a roadway, or in following a curve or grade in the roadway. Particularly to assist a driver of a vehicle at night, these markers have light reflectors which catch and return pencils of light from vehicle headlights back toward the source of the light. Since automobiles of recent vintage have quite powerful headlights, the use of roadmarkers has become more widespread. Roadmarkers contribute to traffic safety such as when roads are wet from rain. Under certain conditions, such as fog, roadmarkers can be the only means of orienting a driver to a changing direction of a road.
Many forms of light reflectors have been suggested. They usually suffer from one or more limitations, such as reflecting too small a proportion of incident light while an approaching vehicle is still at an appreciable distance. As a result, reflecting markers are often noticed too late by a driver to be of substantial help.
Further, in order to avoid making a roadmarker an obstruction on the road, the marker preferably is designed to protrude from the road only a slight amount. This requirement augments problems of light reflection. Plain ceramic or plastic markers have been used, but they tend only to scatter the light. Light scattering is self-defeating in that it is accompanied by losses of intensity of the reflected light which materially reduce the effectiveness of the marker.
An effective reflecting system that has become increasingly popular is a well known triple mirror reflex reflecting principle, such as is disclosed in U.S. Pat. No. 1,671,086 to Stimson, and which is referred to in the art as a cube-corner structure, hereinafter more fully described. While a cube-corner structure provides satisfactory performance as to light striking perpendicularly against an array or strip of cube-corners, that is, parallel to the axes of the cube-corners, this performance falls off rapidly in quality as incident light enters at angles away from the normal to the surface of the cube-corner array. Accordingly, it was previously thought impractical to use a cube-corner structure with a collecting lens system where most of the incidental light would strike a cube-corner structure from many angular directions other than one parallel to the cube-corner axes. Additionally, a cube-corner structure is relatively more expensive than other types of light reflectors such as those embodying rounded beads, so that in manufacturing thousands of markers, the total area of a cube-corner array becomes a factor. Consequently, the area of a given tape or strip of a cube-corner array for each marker is generally minimized as much as possible, although the overall performance of a marker may suffer as a result.
While, as indicated, a roadmarker should not extend so far above the level of a road as to become an obstruction to travel, a related problem arises in that a marker of even small vertical elevation tends to accumulate soil, dirt and debris of all kinds around its periphery. When the accumulation blinds the reflector system of the marker, obviously its function is temporarily but effectively destroyed. It is, therefore, important that a roadmarker fairly readily shed any dirt or debris that tends to accumulate over its light reflecting system and protect its reflecting system from road damage.